What tyre pressure?

B

Blue

Active member
Just a quick question, what tyre pressure should I be using for 17" wheels with non-runflats fitted?

I'm about to go check mine in advance of tomorrow after I finish this meal out but just realised I can't remember what the correct pressures are for a fully loaded Z4 running normal rubber :)
 
Best to try a few different ones to see what suits you - tyre pressure varying is one of the cheapest mods you can do (20p at the pump?!).

I currently run 30/32 on my falken f452s, but think it may be a little too soft, some of the sharpness is lost in the turns, so will be topping it up to 32/34 at the weekend to try that.

best,
Si
 
I don't understand why (with a 50 50 weight distribution) we don't have the same at each end but we are running 33 35

It can't be peer pressure.
 
I am very happy to be corrected if anyone thinks I have got the wrong end of the stick here. I just had a quick read around -

Higher pressure gives a smaller contact patch size. The benefits of a high tyre pressure is that the tyres are more predictable and the tyre has less tendency to roll over onto the sidewall during cornering and improved response through left - right - left corners. The benefits of lower pressures are that you get more grip and a smoother ride.

So BMW prefer the zed to have less wobbly rear end and more grip at the front. I suppose this makes braking and steering easier to control in normal cornering as it isn't necessary to worry what is happening behind you.

I think I shall stick with higher pressures at the back.
 
Just to ask, when I first picked up my car the previous owner had all four tyres under 20psi...
Is it correct to go with 2.0 bar fronts, 2.1 Bar backs on 19" run-flats?
 
Finisterre said:
Higher pressure gives a smaller contact patch size. The benefits of a high tyre pressure is that the tyres are more predictable and the tyre has less tendency to roll over onto the sidewall during cornering and improved response through left - right - left corners. The benefits of lower pressures are that you get more grip and a smoother ride.
Click to expand...
All true, but might be over simplified. I stand to be corrected as well. Lower pressure equating to more grip only applies uniformly to forward/reverse traction. At some point for cornering, the rolling sidewall effect takes over the controlling factor for grip and grip decreases. For every load, tyre, temperature, track condition, etc. there is an optimal pressure to maximise cornering forces. Deviating either direction from that results in lower cornering forces. The problem is, some of these conditions are highly variable, so the optimal pressure is a moving target.

Another factor in tyre pressure for many of us is getting an even wear pattern to maximise tyre life. This may or may not be close to optimal traction pressure. Logic says it is: even wear = optimal contact patch = maximum traction.

I think the main reason for higher rear tyre pressure on Zeds is to enhance the propensity to understeer. Engineers consider this the safer condition for less experienced drivers. Caused me to soil my shorts a few times, though. Conventional track advice says you get more understeer with lower front and higher rear pressure. This implies lower pressure yields less traction, which indicates the tyre rolling condition is in effect. This tells me the overall pressures are too low in the first place, so I do not fully understand this track advice.

More discussion please!
 
your last para pretty much sums up my confusion too.

I know from driving a van with a dodgy brake load balance gizmo that you definitely want the rear wheels to lock up after the front wheels, there is only so much sideways that the front wheels can cope with before you spin off or shoot up a bank.

Maybe the wider tyres are part of the design so they can hang on that little bit longer under extreme braking? Still it doesn't make much sense to me to have a smaller area of contact at the back, surely that obviates the wider tyre design to combat sidewall rollover particularly as the rear wheels are cambered to give an oblong footprint under sideways load?

As for understeer, :idunno:

in fact
:idunno: :idunno: :idunno:
I think we probably need a list of forces and situations with a spreadsheet of results before we can see the logic of rear wheel tyre pressures.
 
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